1. Field of Invention
This invention relates in general to the field of Internet Protocol (IP) multicast. More specifically, the invention relates to methods and systems for determining Path Maximum Transfer Units (PMTU) in an IP multicast.
2. Description of the Background Art
An Internet Protocol (IP) multicast is a network comprising a source and a plurality of destinations. Source is a network node from which data is transferred to one or more destinations. In a network, the source and destinations are connected to each other via a series of paths. The path includes one or more router. For example, a source A may be connected via a path to a router B. The router B may be connected to another router C. The router C is then connected to three destinations—D, E and F.
A packet is transmitted by a path in the network only if the size of the packet is less than or equal to the Maximum Transfer Unit (MTU) of the path If the size of the packet is larger than the MTU of the path, the router fragments the packet. However, fragmentation of packets at routers is not preferred since it affects the performance of the network In an IP multicast, fragmentation affects the performance of the node carrying out the fragmentation and the destinations receiving the fragmented packets. To prevent fragmentation, a Don't Fragment (DF) bit is set on the packets.
When a router is unable to transmit a packet because its size exceeds the MTU of the path and its DF bit is set, the router sends back a message to the source, stating ‘Destination Unreachable, DF bit set’. The message is sent using Internet Control Message Protocol (ICMP) and the message is called ‘Packet too BIG’ ICMP message. The router sending the message also includes the MTU of the path on which the fragmentation failed. In response to the ‘Packet too BIG’ ICMP message, the source starts transmitting the packet of a smaller size. Usually the size of the packet is taken to be equal to the MTU received in the ‘Packet too BIG’ ICMP message.
However, if a packet transmitted by the source is destined for a plurality of destinations, such as in an IP multicast, the message is not sent back to the source. This is to avoid an implosion of traffic at the source. The packets are simply dropped, i.e., they are not sent to the destination devices. This prevents the source from modifying the size of the packet, to ensure that subsequent packets are of a size that is smaller than the MTU of the path.